1. Field of the Invention
The present invention relates to vehicle front end suspension systems and, more particularly, to an air spring based suspension system directed to improving vehicle stability by reducing front end dive during braking and increasing resistance to vehicle roll during cornering.
2. Background
Truck suspension systems are designed to meet each of several not wholly compatible goals which include: absorbing road shock and providing a comfortable ride; stabilizing the vehicle, especially during cornering and braking, to help the driver keep control of the vehicle; and maintaining proper axle spacing and alignment, which also helps to keep the vehicle under control and extend tire life. These goals must met while supporting the vehicle""s weight over a wide range of vehicle load conditions.
There are four basic categories of suspension systems used on trucks: leaf spring systems; equalizing beam systems; torsion bar systems; and air spring systems. The categories are not mutually exclusive and elements of more than one system may be combined to build a hybrid suspension system, at some cost both in terms of money and complexity.
Air spring systems have recently gained in popularity and have been applied to a variety of truck axles, including of particular interest here, the steering axle. Air spring suspensions give excellent load and vibration isolation to the cab by eliminating the interleaf friction found in traditional multiple leaf spring designs. The deflection rate of air springs can be adjusted automatically to compensate for vehicle load changes. As a result, vehicle height does not vary with load or positioning of the load, thereby enhancing driver control. In addition, an air spring usually has a lower deflection rate than a leaf spring exerting the same force giving the system greater capacity for absorbing shocks for a given displacement between the axle and the frame.
Air springs are also employed to maintain a constant vehicle height despite changes in vehicle loads. As such it may sound odd to refer to a deflection rate for such springs since the deflection rate for a compression spring equates spring deflection with force generated. Air springs, unlike conventional springs, can be and are used to generate a varying amount of force while maintaining a fixed height displacement. This is effected by changing air pressure in the air spring in response to changes in vehicle height, either dumping or adding air to the air spring by valves. Conventional springs must of course deflect to generate a balancing counter force. In effect, as air pressure is changed in an air spring in order to maintain a constant height, the deflection rate of the spring is changing. Thus, air springs may be termed controllable rate springs or controllable deflection springs.
In an air spring based system, air bellows are positioned with respect to an axle and a vehicle frame to support the frame from the axle. The air spring can be used to supplement a leaf spring arrangement by being placed between the leaf spring and the vehicle frame. Commonly though, air spring systems replace the leaf spring. In a typical application of air springs to a steering axle, an air spring is placed adjacent each wheel over the axle and directly below the side rails of the vehicle frame.
Pure air spring based systems are not without problems. Air springs, for all of their advantages in providing a comfortable ride and adaptability to changing load conditions, have required substantially more complex and costly suspension designs than have leaf springs. A leaf spring provides two frame mounting points fore and aft of the steering axle to aid in axle stabilization and location, whereas an air spring provides nothing in the way of axle stabilization and location. An air spring suspended steering axle has typically been stabilized using trailing connecting rods or arms between the frame rails and the steering axle. A lateral track bar has provided lateral stabilization for the axle. Trailing arm systems achieve substantial front end anti-roll stiffness by positioning rigid arms between the frame and the axle, with each arm being pivotally attached to the frame and rigidly attached to the axle. The trailing arm design used with air springs at the vehicle front end is not without disadvantages. During vehicle braking, the front ends of vehicles tend to dive. In traditional leaf spring suspension designs, where the leaf spring is mounted to the frame at two points, ahead of the solid axle and following the axle, the torque reaction force generated by the brakes on the axle in turn generates a reactive upward force on the frame through the leaf springs aft mounting point and a downward reactive force through the forward mounting point. No net downward force is transmitted from braking. In trailing arm/air spring suspension designs this balance is lost. Trailing arm designs transmit the brake reaction torque to the frame only through the forward trailing arm mount as a downward force and thereby increase dive. Most trailing arm designs are also poor at maintaining axle position laterally, necessitating the use of a lateral track bar to hold axle position.
It is an object of the invention to provide an air spring suspension system with improved vehicle stability characterized by increased resistance to front end dive on braking and improved resistance to roll.
It is another object of the invention to provide a suspension system which enhances steering axle lateral stability without use of a track bar.
The foregoing objects are achieved as is now described. The invention provides a suspension system for the steering axle of a vehicle chassis. The suspension includes a right side air spring and a left side air spring. Both air springs are mounted above the axle and below their respective sides of the vehicle chassis for supporting the chassis. Auxiliary axle stabilizing and locating apparatus include pairs of hanger brackets depending from the vehicle chassis forward of the steering axle, one on each major side of the vehicle chassis and additional pairs of hanger brackets depending from the vehicle chassis aft of the steering axle, one on each major side of the vehicle chassis. Shackle linkages are coupled between forward hanger brackets. A right side spring half leaf and a left side spring half leaf are mounted between the frame rails and opposite ends of steering axle. Each spring half leaf is pivotally connected at one end to shackle linkages and at the opposite end is rigidly mounted on the steering axle beneath the air springs. A right side rigid arm and a left side rigid arm complete positioning of the axle. Each rigid arm is pivotally coupled at one end to hanger brackets, depending from a frame rail, and at the opposite end rigidly attached to the steering axle below an air spring. The rigid-arms are preferably connected as leading arms, but may be mounted as either leading or trailing arms, depending on the location of a steering linkage to the axle.
Additional effects, features and advantages will be apparent in the written description that follows.